The present invention relates to a method for detecting a malfunction during drilling operations, by making use of two indicators.
In the field of drilling operations, especially in the oil drilling sector, it is a known practice to carry out an analysis of the malfunctions in a drilling operation by means of the analysis of the mechanical specific energy applied in practice during the drilling operation. However, the precision of such known analyses do not appear to be satisfactory. In particular, these known analyses do not provide the ability to differentiate between the passage of the drilling tool from a formation that is softer to formation that is harder and the presence of an actual malfunction during the drilling operation. Furthermore, when malfunction, whether or not real, is identified by making use of these known analyses, the degree of precision and accuracy of these tests is insufficient to enable any determination of the specific type of malfunction that has been encountered in the drilling process.
There is therefore a need to develop a more precise method for detecting a malfunction in drilling operations. To this end, the invention provides a method for detecting a malfunction during a drilling operation carried out by making use of a drill bit, the method including the following:
a) the comparison of a first magnitude representative of the mechanical specific energy, with a first threshold value;
b) when the first magnitude is greater than the first threshold value, the comparison of the ratio between the first magnitude and a second magnitude representative of the drilling force with a second threshold value;
c) the detection of a malfunction in the drilling operation when the ratio between the first magnitude and the second magnitude is greater than the second threshold value.
According to one variant embodiment, prior to the steps a) and b), the method includes the providing of an estimate of at least one of the following variable values:                the rate of penetration of the drill bit;        the weight applied on the drill bit; and        the torque applied to the drill bit;        
the first magnitude being calculated based on the torque applied to the drill bit and the rate of penetration of the drill bit; and
the second magnitude being calculated based on the weight applied on the drill bit and the rate of penetration of the drill bit.
According to one variant embodiment, the first magnitude is obtained by using the following equation:
  E  =            120      *      π      *      RPM      *      TOB                      A        B            *      ROP      where RPM is the number of revolutions per minute of the drill bit;
TOB is the torque applied to the drill bit;
ROP is the rate of penetration of the drill bit;
AB is the surface area of the bore hole.
According to one variant embodiment, the estimate of the torque applied to the drill bit is obtained independently of the weight applied on the drill bit. According to one variant embodiment, the comparisons with the first and second maximum threshold values are performed in real time.
According to one variant embodiment, the method includes, following conclusion of the step c):
d) the determination of a malfunction of the type related to lateral vibration of the drill bit when the variations in the torque applied to the drill bit are lower than a third threshold value.
According to one variant embodiment, the method includes, following conclusion of the step d):
e1) the determination of a malfunction of the type related to the axial vibration of the drill bit when the variations in the weight applied on the drill bit are greater than a fourth threshold value.
According to one variant embodiment, the method includes, following conclusion of the step d):
e2) the determination of a malfunction of the type related to torsional vibration of the drill bit when the variations in weight applied on the drill bit are lower than a fourth threshold value.
According to one variant embodiment, the method includes, following conclusion of the step b), when the ratio between the first magnitude and the second magnitude is lower than the second threshold value:
f) the comparison of the ratio between the first magnitude and the second magnitude with a fifth threshold value;
g) the detection of a malfunction when the ratio between the first magnitude and the second magnitude is lower than the fifth threshold value.
According to one variant embodiment, the method includes, following conclusion of the step g):
h1) the determination of a malfunction of the type related to balling of the drill bit when after the cleaning of the drill bit, the ratio between the first magnitude and the second magnitude is greater than the fifth threshold value.
According to one variant embodiment, the method includes, following conclusion of the step g):
h2) the determination of a malfunction of the type related to wear and tear of the drill bit when after the cleaning of the drill bit, the ratio between the first magnitude and the second magnitude remains lower than the fifth threshold value.